Asphalt is a common material utilized for the preparation of paving and roofing materials. A wide variety of compounds have been added to asphalt surfacing compositions in an attempt to improve the adhesion/wetting of the asphalt to the aggregate.
A wide variety of silicon compounds including silanes have been used for the impregnation and surface treatment of glass fibers to promote the adhesion of various organic resins such as the polyester resins, epoxide resins or phenol formaldehyde resins and for the surface treatment of textiles, leathers, ceramics and glass materials.
It has been found that the adhesion of asphalt to siliceous surfaces could be markedly improved by first treating the surfaces with vapors of a mixture of methylchlorosilanes. However, this exotic method of applying silanes to the aggregate surface is not practical in large scale applications. Sanderson, F. C., “Methylchlorosilanes as Anti-stripping Agents”. Proceedings, Highway Research Board, 31, 288 (1952).
U.S. Pat. No. 2,570,185 discloses that the coating properties and anti-stripping properties of asphalt is improved by addition of the reaction product of aminoalkoxysilanes and high molecular weight aliphatic primary amines containing at least 6 carbon atoms to asphalt. The only example of a silane shown in U.S. Pat. No. 2,570,185 is di-t-butoxy-diamino silane. U.S. Pat. No. 2,985,678 discloses that higher alkyl or aryl radicals in silicon compounds progressively lower the stability of the compounds. However, the tertiary butyl radical is shown to increase the stability of the silicon compounds, even in silicon compounds which contain long chain alkyl such as a lauryl group.
German Patent No. 800,685 teaches silanes of the formula SiRm Xn as asphalt adhesion agents, wherein X represents a halogen or an alkoxy, R represents an organic residue and m and n represent integers of from 1 to 3. Specific examples of R include methyl, phenyl and 2-chloroethylene. U.S. Pat. No. 4,036,661 discloses using a variety of organofunctional silanes as adhesion promoters for asphalt-mineral compositions. U.S. Pat. No. 5,130,354 discloses using silane functionalized polymers as adhesion promoters for asphalt concrete mixtures for paving. U.S. Pat. Nos. 4,170,484 and 4,038,096 disclose using silanes to improve adhesion of asphalt binders to mineral aggregate.
The thermal stability of silicon compounds, however, is also of primary importance. That is, it is highly desirable that the compounds not only promote the adhesion of asphalt to mineral aggregate but also remain stable over a wide temperatures range and for an extended period of time. In addition to this, it is highly desirable that the adhesion promoters be capable of being used without exotic application methods. To be suitable for paving materials, the asphalt-silicon compound mixtures should be mixable in asphalt and stay mixed during subsequent processing. The compound must be stable to oxidation during processing and during long term aging due to weathering. Stability at temperature over 180° C. of silicon compound modified asphalt is a major consideration. Lower vapour pressure, higher boiling point and oxidative stability at 150° C.-180° C. are important for improved performance.
One drawback of using organosilanes is their inability to exhaust and react completely with the surfaces of aggregates if mixed with asphalt. As such, pre-treatment of aggregates with a silane is always required to achieve the desired efficacy of the adhesion promoters. Pre-treatment of aggregates is not practical and is very expensive to practice.
In addition to silanes, long chain tertiary amines and quaternary amines have traditionally been used as additives in asphalt to improve the adhesion of asphalt concrete. These products perform adequately at 0.5%-3% by weight of the asphalt. These compounds, however, have the disadvantage of increasing rutting and have poor fatigue resistance and function by mechanism of wetting and physical bonding. Additionally, due to availability and sustainability issues and the need to minimize consumption and increasing lack of good quality of aggregates and asphalt, there remains a need for asphalt-mineral compositions that have further improved adhesion and/or reduced susceptibility to moisture.